1. Field of the Invention
The present invention relates to a micro-oscillation element having a minute oscillating portion, such as a micromirror element, an acceleration sensor, and an angular speed sensor.
2. Description of the Related Art
Lately in various technical fields, application of elements with a minute structure formed through a micromachining technique has come to be focused on. Such elements include a micro-oscillation element that has a minute movable portion or oscillating portion, such as a micromirror element, an acceleration sensor, and an angular speed sensor. The micromirror element is employed for reflecting light, in a technical field related to an optical disk or optical communication, for example. The acceleration sensor and the angular speed sensor are employed, for example, for controlling posture of a robot or stabilizing an image in a digital camera against the user's hand motion.
The micromirror element includes a mirror surface that reflects light, so that oscillating the mirror surface can change the reflection direction of the light. Most apparatuses employ a static drive type micromirror element, which utilizes a static power for oscillating the mirror surface. The static drive type micromirror element may be broadly classified into one processed by a so-called surface micromachining technique and another processed by what is known as a bulk micromachining technique.
The surface micromachining technique includes processing material thin films corresponding to each region constituting a chip in a desired pattern on a substrate, and sequentially stacking such patterns to thereby form each component constituting the chip such as a supporting body, the mirror surface and electrodes, and a sacrifice layer which is to be removed later. The bulk micromachining technique includes etching the material substrate itself, thereby forming the supporting body and the mirror base in a desired pattern, and forming thin films that serve as the mirror surface or the electrode, as the case may be. The bulk micromachining technique is described, for example, in JP-A-H10-190007, JP-A-H10-270714, and JP-A-2000-31502.    Patent document 1: JP-A-H10-190007    Patent document 2: JP-A-H10-270714    Patent document 3: JP-A-2000-31502
Technical requirements of the micromirror element include high flatness of the mirror surface engaged in reflecting light. Whereas, in the case of employing the surface micromachining technique, the mirror surface is prone to be bent because the finished mirror surface is very thin, and it is hence quite difficult to secure the required flatness over the mirror surface having an extensive area. By the bulk micromachining technique, on the other hand, a relatively thick material substrate is processed by etching to form a mirror base, upon which a mirror surface is formed. Accordingly, the mirror surface can retain sufficient rigidity despite having a wide area. Consequently, the bulk micromachining technique provides the mirror surface with sufficiently high optical flatness.
FIGS. 26 and 27 illustrate a conventional static drive type micromirror element X4 processed by the bulk micromachining technique. FIG. 26 is an exploded perspective view of the micromirror element X4, and FIG. 27 is a cross-sectional view taken along a line XXVII-XXVII of the micromirror element X4 in FIG. 26, based on the assembled state.
In the micromirror element X4, a mirror substrate 40 is stacked on a base substrate 46. The mirror substrate 40 includes a mirror base 41, a frame 42, and a pair of torsion bars 43 connecting the mirror base 41 and the frame 42. Performing an etching process on either side of a substrate of a predetermined conductive material, such as a silicon substrate, can lead to formation of the outer shape of the mirror substrate 40 including the mirror base 41, frame 42, and the pair of torsion bars 43. On the upper face of the mirror base 41, a mirror surface 44 is provided. On the back of the mirror base 41, a pair of electrodes 45a, 45b is provided. The pair of torsion bars 43 defines an axial center A4 of the rotating motion of the mirror base 41, which will be subsequently described. The base substrate 46 includes an electrode 47a facing the electrode 45a of the mirror base 41, and an electrode 47b facing the electrode 45b. 
In the micromirror element X4, when a potential is applied to the frame 42 of the mirror substrate 40, the potential is transmitted to the electrode 45a and the electrode 45b via the pair of torsion bars 43 and the mirror base 41, which are integrally formed with the frame 42 from the same conductive material. Accordingly, applying a predetermined potential to the frame 42 allows charging the electrodes 45a, 45b positively, for example. When the electrode 47a of the base substrate 46 is negatively charged under such state, a static attractive force is generated between the electrode 45a and the electrode 47a, thereby causing the mirror base 41 to rotate in a direction indicated by arrows M4 as shown in FIG. 27, twisting the pair of torsion bars 43. The mirror base 41 can oscillate in an angle where the static attractive force between the electrodes and a total sum of the torsional resistance of the respective torsion bar 43 are balanced. On the other hand, negatively charging the electrode 47b while the electrodes 45a, 45b of the mirror base 41 are positively charged generates a static attractive force between the electrode 45b and the electrode 47b, thereby causing the mirror base 41 to rotate in a direction opposite to the arrows M4. Driving thus the mirror base 41 to oscillate allows switching the direction of light reflected by the mirror surface 44.
For the micro-oscillation element having an oscillating portion, the natural frequency or resonance frequency relevant to the oscillating motion of the oscillating portion is a critical characteristic that definitely determines the motion speed and oscillation amplitude (maximum oscillation angle) of the oscillating portion. In the conventional micro-oscillation element, in order to adjust the natural frequency of the oscillating portion after once completing the formation of the chip, it is necessary to perform a trimming process with a laser or focused ion beam on the oscillating portion thereby scraping the oscillating portion thus to reduce the mass, hence the inertia thereof, or to perform a trimming process on a link portion connecting the oscillating portion and the frame (immobile portion) thereby scraping the link portion thus to reduce the torsion spring constant thereof (because generally the smaller the inertia of the oscillating portion is, the higher the natural frequency thereof is, and the smaller the torsion spring constant of the link portion is, the lower the natural frequency thereof is). In order to adjust the natural frequency of the mirror base 41 (oscillating portion), for example in the micromirror element X4, it is necessary to perform the trimming process either on the mirror base 41 thereby reducing the inertia of the mirror base 41, or on the torsion bars 43 connecting the mirror base 41 and the frame 42 (immobile portion) thereby reducing the torsion spring constant of the torsion bar 43. Adjusting the natural frequency of the oscillating portion after once completing the formation of the chip is particularly necessary when collectively processing identically designed micro-oscillation elements on the wafer thus executing a mass production. This is because, in the case of the mass production, fluctuation in natural frequency among the chips is prone to be incurred from an error in processing dimensions in the oscillating portion or the link portion.
Such adjustment of the natural frequency by a posterior mechanical process (trimming process), however, incurs an increase in the number of manufacturing steps of the micro-oscillation element, as well as in manufacturing cost thereof. Besides, such posterior mechanical process only allows reducing the inertia of the oscillating portion or the torsion spring constant of the link portion for the adjustment of the natural frequency, thereby restricting of freedom in adjusting the natural frequency of the oscillating portion.